1. Field of the Invention
The present invention generally relates to lasers, and particularly to spectral control systems for lasers that generate a narrow-band laser output.
2. Description of Related Art
Lasers are employed in a wide variety of uses: medical devices, communications, scientific research, holography, and laser light shows, for example. Generally, a laser is a device that emits high intensity monochromatic optical radiation, usually as a highly directional beam.
Broadly speaking, a laser device includes a gain medium situated within a laser cavity defined by two end mirrors and a pump source used to pump the gain medium to an energy state sufficient to support lasing operation. Many types of lasers have been developed: solid state lasers that utilize an optically-pumped gain media such as Nd:YAG, gas lasers in which a gas such as HeNe or argon is disposed within an electrical discharge tube, and semiconductor lasers that are pumped by an electrical current applied directly across the semiconductor material.
One important characteristic of a laser is its lasing wavelength, which is the wavelength of the fundamental laser emission within the laser cavity. The possible lasing wavelengths are determined by the particular gain medium implemented in a laser. Some gain media lase only on discrete transitions; for example a neodymium-doped solid state gain medium such as Nd:YAG may lase at any of a number of transitions such as 1064 nm, 1123 nm and 947 nm, and argon gas may lase at any of ab number of lines. Other gain media are broadband; for example Ti:sapphire (Ti:Al2O3) is a tunable solid state gain medium, chromium-doped solid state gain media such as Cr:LiSAF laser lase over a broad band of wavelengths that may extend 100 nm or more, and dyes are tunable over a range of about 20–70 nm.
For some gain media and in some simple low loss laser configurations, the strongest lasing transition will usually dominate the fundamental emission; for example the 1064 nm transition in Nd:YAG is strong enough that it will usually dominate the other nearby transitions. However for effective lasing operation and to provide a useful laser output (e.g. to select a transition other than the strongest transition, or to narrow the linewidth of the laser output) it is usually necessary to restrict the lasing wavelength in some way; i.e. some type of spectral control is usually required.
Accordingly, many lasers incorporate some type of spectral selection system in order to control the spectral content of the laser emission within the laser cavity. One common spectral selector is an etalon, which comprises an optical material that has two opposing parallel optical surfaces with a finite reflectivity. An etalon can be useful to narrow the linewidth and select a particular transition in a gain medium that lases on discrete transitions; however, for broadly-tunable gain media an etalon cannot select a single wavelength; instead it selects a series of lasing wavelengths within the gain-bandwidth of the particular gain medium. It is known that an etalon selects a periodically-repeating series of maxima determined by FSR of the etalon; and practical limitations prevent a FSR greater than about 15 nm @1 micron.
Another type of spectral selection system is a wavelength-selective reflective mirror. For example, U.S. Pat. No. 4,615,034 to von Gunten et al. discloses a wavelength selective mirror that provides single wavelength operation of a gas laser whose gain medium has discrete transitions; particularly, an output coupler is disclosed that allows oscillation of the 488 nm line of the argon blue/green spectrum while suppressing all other lines in that spectrum. In that application, the filter is functioning as the output coupler of the laser cavity, and requires a well-controlled transmission at the wavelength of interest for the laser to function properly. Additionally, bandwidth and minimum transmission of such a filter are determined by the refractive indices of the coating materials. These two parameters impose different requirements on the relative refractive indices of the materials, which can sometimes be overcome by increasing the number of coating layers. However, for very low transmission optics, the coatings can become too thick, which can significantly compromise performance and make fabrication difficult.
Narrow-band bandpass filters have been used, such as disclosed in U.S. Pat. No. 5,274,661 to select transition lines in gain media by blocking all but one transmission peak. In such narrow bandpass filters, although the objective is to render the transmission spike extremely narrow and with an extremely high transmittance, in practice the maximum reflectance (e.g., 5–10%) still is significantly greater than an AR coating (e.g., <0.2%). Another example of a narrow-band filter is disclosed in U.S. Pat. No. 4,800,568, which discloses a gas ion laser with a Brewster window coated for suppression of unwanted laser frequencies to produce a narrow band Brewster window.
Another common spectral selection system utilizes a birefringent filter (BRF) within the cavity. One example of a birefringent filter includes a birefringent material such as quartz set at Brewster's angle to the optical axis. However, a BRF is highly sensitive to small, normal changes in the laser cavity. Furthermore, in some lasers such as frequency-converted lasers, the presence of other birefringent materials (which may be intentionally made with small wedge angles) in the cavity makes such a BRF even more difficult to implement. Also, because the wavelength selectivity of a BRF is highly sensitive to beam divergence, any beam divergence unfortunately broadens the bandwidth, and thus use of a BRF is difficult to implement in a compact solid-state laser cavity. Particularly, any divergence of the beam causes the optical path length through the BRF to vary transversely across the beam; accordingly the wavelengths of the peak transmission will also vary transversely across the beam. Therefore, for all these reasons, a BRF has serious limitations that prevent its effective utilization.
Still another common spectral selection method utilizes one or more prisms situated within an optical cavity. Such prisms have long been used on argon ion lasers for example. Although prisms can be effective in long cavities (e.g. >20 cm), prisms are not a practical solution in smaller, highly diverging cavities because the beam divergence is greater than their dispersion. Particularly, prisms are simply not wavelength-selective enough in compact cavities. Furthermore, a prism system can be expensive, can cause temperature stability problems, and adds to the complexity of the laser device.
The above-described spectral control systems are useful to select a desired lasing wavelength and/or to narrow the linewidth. However, even with these spectral control systems many wavelengths are simply not achievable in a practical, cost-effective laser due to gain-bandwidth limitations of the gain medium and practical pump source limitations, particularly within small length and size limitations that are attractive to users. For example, laser diodes are currently the most practical pumping sources for solid state lasers. However, the available wavelengths of laser diodes suitable for pumping are limited, and because in most practical lasers the available output wavelengths are restricted by the pumping wavelength, the laser diode wavelength limitations also limit the available laser emission, which correspondingly restricts the available output wavelengths of non-frequency converted lasers.
To the end user, the output wavelength can be important; for example a short wavelength (e.g. blue) is more useful for creating compact discs (CDs) than a longer wavelength (e.g. infrared). Additionally, many biosciences applications utilize dyes that are only sensitive within certain narrow wavelength regions. In order to expand the available laser wavelengths, 1) tunable gain media have been developed, and 2) frequency conversion processes may be employed.
Cr:LiSAF is one example of a broadly tunable laser material (or “gain medium”), with reported laser emission from about 760 nm to about 1000 nm. Although broadly tunable (“broadband”) laser materials generally require careful design to generate a single frequency output, their ability to generate a laser emission at any wavelength within a broad range of wavelengths can be an important advantage. Broadband gain media can be used in single frequency lasers; to obtain single wavelength operation in broadly tunable laser materials such as Cr:LiSAF, spectral selection methods such as a BRF may be utilized to select a particular wavelength within the wide spectrum under the gain curve. Broadband gain media such as Cr:LiSAF are typically low gain because of the nature of the transitions involved. Additionally, their absorption length is intentionally limited due to thermal and material constraints: too high a doping level can impair the optical quality of the material, and if the gain crystal heats up too much, thermal lensing can adversely affect the cavity and thermal quenching can limit laser activity. However, due to this low gain, it can be difficult to generate effective lasing oscillation, especially at wavelengths away from the peak gain. There are some disadvantages over the typical gain media that operate on discrete transitions For example, broadband materials like Cr:LiSAF and Ti:Al2O3 almost always have lower gain than discrete-transition materials like Nd:YAG, Nd:YVO4, and Yb:YAG, an therefore require a longer gain medium to accomplish a target gain, which complicates the mode-matching requirements to efficiently pump a single (TEM00) lasing mode.
To provide laser emission at wavelengths that are not within the gain-bandwidth of a practical gain media, frequency conversion processes have been utilized. Frequency conversion is provided by a nonlinear element arranged within the laser cavity in a particular configuration. The most common frequency-conversion process is frequency doubling, which halves the wavelength; for example frequency doubling the 1064 nm line of Nd:YAG provides a frequency-doubled output of 532 nm.
Although the frequency conversion process advantageously increases the range of achievable output wavelengths, unfortunately frequency converted lasers are susceptible to severe output instabilities, due at partially to the laser's natural tendency to lase at the wavelength of lowest loss, and nonlinear interactions between multiple wavelength. It has been found that effective spectral selection of the fundamental emission can significantly reduce (or even eliminate) output instabilities in a frequency-converted laser. Particularly, if the spectral selection process can maintain single longitudinal mode operation of the fundamental emission, then the frequency doubling process can occur without (sometimes very large) output intensity variations that could otherwise occur. For some frequency-converted lasers, the spectral selection methods described above may be adequate to provide single frequency operation; for others, these methods may not be adequate or practical. Therefore there is a need for a more effective spectral control system, especially with broadband gain media, as discussed below.
In order to extend the range of available wavelengths, it has been suggested to utilize a broadband gain medium in a non-tunable frequency-doubled laser. It may be noted that, while broadband gain media are tunable, tunability is not a requirement for many lasers; furthermore, tunability of a frequency-converted laser is not feasible because changing the lasing wavelength would also require adjusting the phase matching angle in the LBO crystal, which would be difficult and costly to implement.
One previous design of a single wavelength, frequency-converted laser that uses a broadband gain medium is disclosed in U.S. Pat. No. 6,047,010 (the '010 patent). Specifically, the '010 patent discloses an intracavity doubled Cr:LiSAF laser that has three birefringent elements: a Cr:LiSAF crystal (the laser medium), an LBO crystal (a doubling material), and a birefringent filter (BRF). In one design this laser included a curved input mirror, an unwedged Cr:LiSAF crystal with broadband antireflection (BBAR) coatings, a BRF at Brewster's angle, an etalon, and a singly wedged LBO crystal cut to phase match type 1 near 860 nm with a BBAR on one surface and, on the other surface a high reflectivity (HR) coating at about 860 nm and a BBAR coating at about 430 nm. The wavelength of the laser was substantially determined by the relationship of the phase match cut to the HR surface of the LBO. The BRF was used to constrain laser operation to the wavelength range that could be efficiently doubled by the LBO with its surface functioning as one of the cavity mirrors. Finally, an intracavity etalon was used to provide single frequency, low noise operation.
In practice, this design proved difficult and costly to build. For example, accurately aligning the crystal axes with the Brewster plane of the BRF proved difficult, and furthermore much higher losses than expected were observed after inserting the BRF into the laser cavity. Also, the BRF had a high finesse requirement, and as a result it is technically difficult (or may even be impossible) to suppress unwanted lasing offset from the desired line by one free spatial range of the BRF. Furthermore, in operation this design had a very limited lifetime and therefore proved unreliable due to high sensitivity to very small movements of components. Therefore this design did not provide a practical laser at 430 nm.
It is believed that many of the problems with the laser disclosed in the '010 patent originate from complex interactions of the BRF with other cavity elements. Therefore, it would be advantageous to provide a way to obtain spectral control of the laser output of a frequency-converted laser without using a BRF.
One laser design uses a near-hemispherical cavity to allow a tight focus of the fundamental laser beam in the doubling crystal, and requires a significantly larger beam size in the gain medium to allow efficient mode matching to a highly multimode pump over the long interaction region required to absorb the pump in the Cr:LiSAF. This means that the fundamental laser beam is highly divergent in the region where the BRF would need to sit. This compromises the reflection reduction at Brewster's angle on the BRF, increasing the loss. To avoid this problem with a BRF would require a significantly longer cavity, not possible within certain size constraints.